1. Field of the Invention
The present invention relates to apparatus for the detection of seismic waves using geophones as well as to methods for recording and processing the waves detected thereby, and more particularly relates to apparatus capable of detecting both shear and compressional seismic waves and to methods for obtaining shear and compressional seismic wave traces using such apparatus.
2. Description of the Prior Art
In the search for petroleum and other valuable resources, it has become the practice to transmit a seismic signal into the earth from one or more sourcepoints near the surface of the earth. The reflected and/or refracted energy returning from within the earth to a receiver location is detected by a geophone or other seismometer and raw seismic data is recorded. The raw seismic data is mathematically processed and then interpreted to provide an indication of the structure of the underlying strata.
Geophones that detect the reflected seismic signals are generally electromagnetic devices that contain at least one coil representing the electric circuit and at least one magnet which cooperates with the coil. Either the coil or the magnet is rigidly connected to a frame, while the other is elastically suspended from the frame. In response to earth motion, all of the fixed elements of the geophone move with the frame, but the suspended element, under the influence of inertia, tends to be delayed with respect to the unsuspended element, creating relative movement between the coil and magnet. This movement induces an electrical current in the coil which comprises the output signal of the geophone. In any of these geophones, there exists a primary axis, such as a horizontal axis or a vertical axis, along which the suspended element is free to reciprocate, thereby giving rise to the electrical output signal.
Geophones have been used to detect different types of generated seismic signals during seismic exploration of earth strata. The most common type is the so-called compressional (P) wave in which the earth particle motion is in the same direction as the wave propagation. Compressional waves are also sometimes called longitudinal waves. The other type is the shear wave in which the earth particle motion is normal to the direction of wave propagation. Shear waves in which the particle motion is oriented normal to the incident plane are called horizontal shear (SH) waves and shear waves in which the particle motion is oriented within the incident plane are called vertical shear (SV) waves.
Compressional waves are the signals most commonly used for seismic exploration. Compressional waves may be generated in numerous ways, such as the detonation of an explosive, the dropping of weights or the use of mechanical vibrators, and compressional waves are usually detected by so-called vertical geophones designed and manufactured with a vertical primary axis for detecting vertical earth movement. Recently, however, there has been an increased interest in the use of shear waves in seismic exploration. U.S. Pat. Nos. 3,286,783 to Cherry et al., 3,302,164 to Water et al., 3,372,770 to Clynch, 3,835,954 to Layotte, 3,736,556 to Barr, Jr , and 4,059,820 to Turpening disclose various apparatus and methods for the generation, detection, and use of shear waves in seismic exploration. Horizontal shear waves have generally been detected using horizontal geophones.
It has been proposed to use a combination of a vertical geophone and one or more horizontal geophones mounted on a common baseplate or a plurality of closely grouped base plates. However, a need exists for alternatives to such geophone apparatus which result in greater accentuation of the desired signal components, i.e., either the compressional wave components or the horizontal shear wave components, and greater attenuation of the undesired components.
Accordingly, it is a primary object of this invention to provide a geophone apparatus capable of detecting both horizontal shear and compressional seismic waves, the use of which apparatus will result in the production of seismic traces with improved quality.
Another object of this invention is to provide a geophone apparatus capable of detecting both horizontal shear and compressional seismic waves and which is fully compatible with the other apparatus conventionally employed in seismic exploration.
A further object of this invention is to provide a seismic exploration method for concurrently detecting both horizontal shear and compressional seismic waves and for processing the data obtained to produce a compressional wave trace and, optionally, a horizontal shear wave trace of improved quality.
Other objects, advantages and features of this invention will become apparent to those skilled in the art from the following description when taken in conjunction with the drawings.